Put the pieces of white chocolate in a bowl, and sit this bowl over a pan over simmering water until it melts, stirring gently with a spatula every now and then. When it's melted, stand the bowl on a cold surface to cool down a little.
In another bowl, and using an electric handheld whisk for ease, whip the cream, egg white and extract together. You want soft peaking rather than a stiff mixture.
Put a big dollop of cream onto the slightly cooled chocolate and mix, and then gently fold the chocolate mixture into the cream.
Divide mixture between 6 small but perfectly formed glasses with a capacity of 60ml/ 1/4-cup each.
Chill in the fridge or give them a fast icy zap but sitting them in the deep freeze for 10 or 15 minutes. Decorate top with a mint leaf before serving.